Prepare API for upcoming UI changes

[ganhongyao]

Changes

• Add primary donor logo (frontend + backend)
• Add campaign image in interest (frontend + backend)
• Replace is_redeemed column in Coupons table with campaign_charity_id
  • This is because the new schema is such that if a coupon is redeemed without any personal contributions, there are no secondary donations. We need campaign_charity_id to keep track which campaign charity this coupon was redeemed for.
  • This makes redeemed? derived from whether the campaign_charity_id is null or not
• Rework secondary_donations POST endpoint

Behaviour	Endpoint	Request body type
Redeeming a coupon with personal contribution	POST coupons/redeem	CouponRedeemPostData
Redeeming a coupon without personal contribution	POST coupons/redeem	CouponRedeemPostData, with amount = 0
Making an external contribution without coupon	POST secondary_donations	SecondaryDonationPostData

• Update views as necessary for upcoming UI changes

[sivayogasubramanian]

I am going to request changes as I come across them when I am doing the frontend components.

frontend/components/campaigns/campaignCharities/CampaignCharityCard.tsx in #252 would need PrimaryDonor name in the CampaignCharityDonationPublicData.

[sivayogasubramanian]

I made the fixes in #252. Help check if its okay.

[sivayogasubramanian]

LGTM! Didn't really test everything as it will be easier to find issues during integration.